Use the faster container based travis

As per http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/ they have new docker-based infrastructure which is much quicker at running tests.

As we don't use sudo to install any packages as root, we can take advantage of this by setting `sudo: false` in our travis config. The last build I did before this took ~9 minutes, the build for this branch took ~4 minutes. Nice easy speedup!